Helium (chemical symbol He, atomic number 2) is a minor component of the Earth's atmosphere, but it is the second most abundant element in the universe and second lightest of all known elements. It is a colorless, odorless, tasteless, nontoxic, and nearly inert gas that heads the noble gas series in the periodic table. Its boiling and melting points are the lowest among the elements, and extreme conditions are needed to convert it into the liquid and solid forms. Extreme conditions are also needed to create the small handful of helium compounds, which are all unstable at ordinary temperatures and pressures.

In the present-day universe, almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. On Earth, it is produced by the radioactive decay of much heavier elements. After its creation, part of it is trapped with natural gas, at concentrations of up to 7 percent by volume.

It is commonly known that helium is used for providing lift for balloons and airships. In addition, it is used as a component in deep-sea breathing systems, as a coolant for superconducting magnets, and as a protective gas for many industrial processes such as arc welding and growing silicon wafers. Researchers use helium to study materials at very low temperatures, in a field called cryogenics, and in helium dating of radioactive rocks and minerals. Inhaling a small volume of the gas temporarily changes the tonal quality and pitch of one's voice. It can, however, be dangerous if done in excess.

Abundance in nature

Helium is the second most abundant element in the known universe, after hydrogen, constituting 23 percent of the elemental mass of the universe. It is concentrated in stars, where it is formed by two sets of nuclear fusion reactions: one involving the "proton-proton chain reaction" and the other involving the "carbon-nitrogen-oxygen cycle." According to the Big Bang model of the early development of the universe, the vast majority of helium was formed between one and three minutes after the Big Bang, at a stage known as the Big Bang nucleosynthesis. Based on this theory, the abundance of helium serves as a test of cosmological models.

In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million, largely because most helium in the Earth's atmosphere escapes into space due to its inertness and low mass. In the Earth's heterosphere (a part of the upper atmosphere), helium and other lighter gases are the most abundant elements.

Nearly all helium on Earth is a result of radioactive decay. The decay product is found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite, monazite and beryl. These minerals emit alpha particles, which consist of helium nuclei (He2+), to which electrons readily attach themselves. In this way, an estimated 3.4 liters of helium are generated per year per cubic kilometer of the Earth's crust.

The concentration of helium in the Earth's crust is 8 parts per billion; in seawater, it is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. The greatest concentrations of helium on our planet are in natural gas, from which most commercial helium is derived.

Scientific discoveries

Pierre Janssen (1824–1907), a French astronomer, was the first to detect evidence of a previously unknown element (helium) in the Sun.

On August 18, 1868, during a total solar eclipse in Guntur, India, French astronomer Pierre Janssen observed a bright yellow line with a wavelength of 587.49 nanometers (nm) in the spectrum of the Sun's chromosphere. This line was the first evidence that the Sun contained a previously unknown element, but Janssen was ridiculed because no element had been detected in a celestial body before being found on Earth. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line of the same wavelength in the solar spectrum. He named it the D3 line (Fraunhofer line), for it was near the known D1 and D2 lines of sodium. He concluded that it was caused by an element in the Sun unknown on Earth. He and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος (helios).

On March 26, 1895, British chemist William Ramsay isolated helium on Earth by treating the mineral cleveite with mineral acids. Ramsay was looking for argon, but after separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright-yellow line that matched the D3 line observed in the spectrum of the Sun.. These samples were identified as helium by Lockyer and British physicist William Crookes. That same year, chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, independently isolated helium from cleveite. They collected enough of the gas to accurately determine its atomic weight.1

In 1907, Ernest Rutherford and Thomas Royds demonstrated that an alpha particle (emitted by radioactive materials) is a helium nucleus. In 1908, Dutch physicist Heike Kamerlingh Onnes was the first to liquefy helium by cooling the gas to below 1 Kelvin (K). He tried to solidify it by further reducing the temperature, but he failed because helium does not have a "triple point" temperature where the solid, liquid, and gas phases are in equilibrium with one another. His student, Willem Hendrik Keesom, was the first to solidify helium in 1926, by subjecting it to a pressure of 25 atmospheres.

In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. In 1972, the same phenomenon was observed with helium-3, by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson.

Notable characteristics

Gas and plasma phases

In the periodic table, helium is at the head of the noble gas series in group 18 (former group 8A), and it is placed in period 1, along with hydrogen. Unlike hydrogen, helium is extremely inert and is the least reactive member of the noble gases. As a result, it is monatomic (consists of single atoms of He) under virtually all conditions.

The boiling and melting points of helium are the lowest among the elements. For this reason, helium exists as a gas except under extreme conditions. Gaseous helium is colorless, odorless, tasteless, and nontoxic. It is less water soluble than any other gas known, and its rate of diffusion through solids is three times that of air and around 65 percent that of hydrogen. The index of refraction of helium (ratio of speed of light in helium to that in a vacuum) is closer to unity than any other gas.

Helium's thermal conductivity (ability to conduct heat) is greater than that of any gas except hydrogen, and its specific heat (amount of energy required to raise the temperature of 1 kilogram of helium by 1 K) is unusually high. At normal temperatures, helium heats up when allowed to expand freely; but below about 40 K (Kelvin), it cools during free expansion. Once it has been cooled below this temperature, helium can be liquefied through expansion cooling.

Helium is an electrical insulator unless ionized. As with the other noble gases, it has metastable energy levels that allow it to remain ionized in an electrical discharge when the voltage is kept below its ionization potential (that is, below the energy required to strip the He atom of an electron).

Helium is chemically unreactive under all normal conditions. Extreme conditions are needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure (0° C and 100 kilopascals pressure).

For instance, helium can form unstable compounds with tungsten, iodine, fluorine, sulfur, and phosphorus when it is subjected to an electric glow discharge, through electron bombardment, or is otherwise a plasma. HeNe, HgHe10, WHe2, and the molecular ions He2+, He2++, HeH+, and HeD+ have been created in this manner. This technique has also allowed the production of the neutral molecules He2 and HgHe .

Throughout the universe, helium is found mostly in a plasma state whose properties are quite different from those of molecular helium. As a plasma, helium's electrons and protons are not bound together, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind together with ionized hydrogen, they interact with the Earth's magnetosphere giving rise to the aurora phenomenon ("Northern lights").

Solid and liquid phases

Unlike any other element, helium fails to solidify and remains a liquid down to absolute zero (0 K) at normal pressures. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) and about 26 standard atmospheres (2.6 MPa) of pressure. It is often hard to distinguish solid from liquid helium because the two phases have nearly the same refractive index. The solid form is colorless and almost invisible; it has a crystalline structure with a sharp melting point; and it is highly compressible—about 50 times more compressible than water.

Helium-4 (the most common isotope of helium) has two different liquid states, helium I and helium II, depending on the temperature. The behavior of these two states is important to researchers studying quantum mechanics (particularly the phenomenon of superfluidity) and those studying superconductivity and other properties of matter at temperatures near 0 K.

Helium I state

Below its boiling point of 4.21 K and above a temperature of 2.1768 K (called the "lambda point" for helium), the helium-4 isotope exists in a normal, colorless liquid state, called helium I. Like other cryogenic liquids, helium I boils when heat is added to it. It also contracts when its temperature is lowered until it reaches the lambda point, when it stops boiling and suddenly expands. The rate of expansion decreases below the lambda point until about 1 K is reached; at which point expansion completely stops and helium I starts to contract again.

Helium I has a gas-like refractive index of 1.026, which makes its surface so hard to see that floats of Styrofoam are often used to show where the surface is. This colorless liquid has a very low viscosity and a density one-eighth that of water, which is only one-fourth the value expected from classical physics. Quantum mechanics is needed to explain this property. For this reason, both types of liquid helium are called quantum fluids, meaning they display atomic properties on a macroscopic scale.

Helium II state

Below the lambda point, liquid helium begins to exhibit very unusual characteristics, in a state called helium II. Helium II cannot be boiled because it has high thermal conductivity (high ability to conduct heat). Instead, when this liquid is heated, it evaporates directly to form gas.

Helium II will "creep" along surfaces in order to find its own level - after a short while, the levels in the two containers will equalize. The Rollin film also covers the interior of the larger container; if it were not sealed, the helium II would creep out and escape.

Helium II is a superfluid, a quantum-mechanical state of matter with strange properties. For example, when it flows through even capillaries of 10-7 to 10-8 m width, it has no measurable viscosity. However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed.

Helium II also exhibits a "creeping" effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region, where it evaporates. It moves in a film that is 30 nm in thickness, regardless of surface material. This film is called a "Rollin film," named after B. V. Rollin, who first characterized this trait. As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches a warmer place and then evaporates.

In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. Superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.

The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper. This is because heat conduction occurs by an exceptional quantum-mechanical mechanism. When heat is introduced, it moves through helium II in the form of waves, at 20 meters per second at 1.8 K, in a phenomenon called second sound.

The isotope helium-3 also has a superfluid phase, but only at much lower temperatures. As a result, less is known about such properties of helium-3.

Isotopes

Although there are eight known isotopes of helium, only helium-3 and helium-4 are stable. The nucleus of helium-3 contains two protons and one neutron, while that of helium-4 contains two protons and two neutrons.

In the Earth's atmosphere, there is one He-3 atom for every million He-4. Helium, however, is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of He-3 is around a hundred times higher. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of 10; this is used in geology to study the origin of such rocks.

The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized nuclei of helium-4. The helium-4 nucleus, consisting of two protons and two neutrons, is unusually stable. It was formed in enormous quantities during Big Bang nucleosynthesis (noted above).

Equal mixtures of liquid helium-3 and helium-4 below 0.8 K will separate into two immiscible phases (two phases that do not mix) due to their dissimilarity (in terms of quantum statistics). Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.

There is only a trace amount of helium-3 on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. In stars, however, helium-3 is more abundant, as a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith (loose material covering solid rock), have trace amounts of helium-3 from being bombarded by solar winds.

The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. These differing isotope abundances can be used to investigate the origin of rocks and the composition of the Earth's mantle.

It is possible to produce exotic helium isotopes that rapidly decay into other substances. The shortest-lived isotope is helium-5, with a half-life of 7.6×10−22 second. Helium-6 decays by emitting a beta particle and has a half life of 0.8 second. Helium-7 also emits a beta particle, as well as a gamma ray. Helium-7 and helium-8 are "hyperfragments" that are created in certain nuclear reactions.

Historical production and uses

After an oil-drilling operation in 1903 in Dexter, Kansas, produced a gas geyser that would not burn, Kansas state geologist Erasmus Haworth collected samples of the escaping gas and took them back to the University of Kansas at Lawrence. There, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas contained, by volume, 72 percent nitrogen, 15 percent methane (insufficient to make the gas combustible), 1 percent hydrogen, and 12 percent of an unidentifiable gas.2 With further analysis, Cady and McFarland discovered that 1.84 percent of the gas sample was helium.3 Far from being a rare element, helium was present in vast quantities under the American Great Plains, available for extraction from natural gas.

This put the United States in an excellent position to become the world's leading supplier of helium. Following a suggestion by Sir Richard Threlfall, the U.S. Navy sponsored three small experimental helium production plants during World War I. The goal was to supply barrage balloons with the non-flammable lifting gas. A total of 200,000 cubic feet (5,700 m³) of 92 percent helium was produced in the program even though only a few cubic feet (less than 100 liters) of the gas had previously been obtained. Some of this gas was used in the world's first helium-filled airship, the U.S. Navy's C-7, which flew its maiden voyage from Hampton Roads, Virginia to Bolling Field in Washington, D.C. on December 7, 1921.

Although the extraction process, using low-temperature gas liquefaction, was not developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craft. This use increased demand during World War II, as well as demands for shielded arc welding. Helium was also vital in the Manhattan Project that produced the atomic bomb.

In 1925, the U.S. government set up the National Helium Reserve at Amarillo, Texas, with the goal of supplying military airships in time of war and commercial airships in peacetime. Helium use following World War II was depressed, but the reserve was expanded in the 1950s to ensure a supply of liquid helium as a coolant when creating oxygen/hydrogen rocket fuel (among other uses) during the Space Race and Cold War. Helium use in the United States in 1965 was more than eight times the peak wartime consumption.

After the "Helium Acts Amendments of 1960" (Public Law 86–777), the U.S. Bureau of Mines arranged for five private plants to recover helium from natural gas. For this helium conservation program, the Bureau built a 425-mile pipeline from Bushton, Kansas, to connect those plants with the government's partially depleted Cliffside gas field near Amarillo, Texas. This helium-nitrogen mixture was injected and stored in the Cliffside gas field until needed, when it then was further purified.

By 1995, a billion cubic meters of the gas had been collected and the reserve was US $1.4 billion in debt, prompting the Congress of the United States in 1996 to phase out the reserve.4 The resulting "Helium Privatization Act of 1996" (Public Law 104–273) directed the U.S. Department of the Interior to start liquidating the reserve by 2005.

Helium produced before 1945 was about 98 percent pure (2 percent nitrogen), which was adequate for airships. In 1945, a small amount of 99.9 percent helium was produced for welding use. By 1949, commercial quantities of Grade A 99.995 percent helium were available.

For many years, the United States produced over 90 percent of commercially usable helium in the world. As of 2004, over 140 million cubic meters of helium were produced annually, with 85 percent of production from the United States, 10 percent from Algeria, and most of the remainder from Russia and Poland. The principal sources in the world are the natural gas wells in the American states of Texas, Oklahoma, and Kansas.

Given that helium has a lower boiling point than any other element, it can be extracted from natural gas by liquefying nearly all the other elements in the mixture, at low temperature and high pressure. The resulting crude helium gas is purified by successive exposures to low temperatures, by which almost all the remaining nitrogen and other gases are precipitated out of the mixture. Activated charcoal is used as a final purification step, usually resulting in 99.995 percent pure helium. The principal impurity in such helium is neon.

Current applications

Because of its low density, helium is the gas of choice to fill airships such as this USGS blimp.

Helium is used for many purposes that take advantage of its unique properties, such as its low boiling point, low density, low solubility, high thermal conductivity, and inertness. A number of these uses are listed below.

As helium is lighter than air, airships and balloons are inflated with helium for lift. In airships, helium is preferred over hydrogen for it is not flammable and has 92.64 percent of the lifting power of hydrogen.

Given its inertness and low solubility in water, helium is a component of air mixtures used in deep-sea breathing systems to reduce the high-pressure risk of nitrogen narcosis, decompression sickness, and oxygen toxicity. For these breathing systems, helium may be mixed with (a) oxygen and nitrogen ("Trimix"), (b) oxygen alone ("Heliox"), or (c) hydrogen and oxygen ("Hydreliox").

The extremely low melting and boiling points of helium make it ideal for use as a coolant in magnetic resonance imaging, superconducting magnets, and cryogenics. Liquid helium is used to produce superconductivity in some ordinary metals (such as lead), allowing for completely free flow of electrons in the metal.

Because helium is inert, it is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, in gas chromatography, and in providing an atmosphere suitable for protecting historical documents. Its inertness also makes it useful in supersonic wind tunnels.

Based on its inertness and high thermal conductivity, helium is used as a coolant in some nuclear reactors (such as pebble-bed reactors) and in arc welding.

In rocketry, helium is used as an ullage medium to displace fuel and oxidizers in storage tanks and to condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to precool liquid hydrogen in space vehicles.

Because it diffuses through solids at a rate three times that of air, helium is useful for detecting leaks in high-vacuum equipment and high-pressure containers.

Precautions

The voice of a person who has inhaled helium temporarily sounds high-pitched, resembling those of the cartoon characters Alvin and the Chipmunks (although their voices were produced by shifting the pitch of normal voices). This is because the speed of sound in helium is nearly three times that in air. Although this effect may be amusing, it can be dangerous if done in excess, because the helium displaces oxygen needed for normal respiration. Unconsciousness, brain damage, and even asphyxiation followed by death may result in extreme cases. Also, typical commercial helium may contain unhealthy contaminants. If helium is inhaled directly from pressurized cylinders, the high flow rate can fatally rupture lung tissue.

Although neutral helium at standard conditions is nontoxic, a high-pressure mixture of helium and oxygen (Heliox) can lead to high-pressure nervous syndrome. A small proportion of nitrogen can alleviate the problem.

Containers of helium gas at 5 to 10 K should be treated as if they have liquid inside. This is due to the rapid and large increases in pressure and volume that occur when helium gas at that temperature is warmed to room temperature.

External links

Helium (at the Helsinki University of Technology; includes pressure-temperature phase diagrams for helium-3 and helium-4)

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